In this business you are an Independent Contractor. In other words you sign up with companies from whom you hope to receive assignments. You watch their job boards and request the assignments that look interesting to you. The job will always tell you what the pay is (fee) and what the reimbursement amount will be for what you must purchase. Once a job is assigned to you, you read the instructions, go to the correct location in the correct time frame (all spelled out in the instructions) and do the required observations. You then collect whatever proofs of visit are required by the instructions (a receipt for purchase, a business card, etc.), go home and enter the report on line. Most companies these days want your proofs of visit scanned and uploaded to them as part of the report. Within the next few days, watch your email in case an editor has questions about your shop. Once accepted, you will be paid for your work usually in 4-6 weeks, with some companies paying faster and some paying slower. It is fairly straightforward though when you are just getting started it seems very confusing.
This is a forum of fellow shoppers. We are here to help you where we can, but getting yourself organized, getting signed up with companies, going through their instructions and getting the work done is all the results of your own efforts. There is a list of legitimate companies in the first post of the first thread of the Mystery Shopping Company Discussion section of the board. Generally we encourage new shoppers to sign up with Corporate Research International and Market Force first as these two companies pay like clockwork (Corporate Research pays every 2 weeks), both companies have nationwide clients so there most likely be work near you, and both companies are fairly new shopper friendly in the clarity of their instructions.